This invention relates to ion mobility spectrometry for gas and liquid sample preparation, filtering, and detection in a field asymmetric waveform ion mobility spectrometer, with electrospray sample delivery, and using either internal or external detectors.
Electrospray mass spectrometry is a powerful analytical tool that has been broadly applied to bio-molecular structure analysis (i.e., Proteins, Peptides and DNA). See Electrospray Ionization Mass Spectrometry Fundamentals, Instruments, and Applications, Richard B. Cole, John Wiley and Sons, 1997. This technique plays a central role in the development of most pharmaceutical drugs and is being used to perform quantitative measurement of human exposure to carcinogens. Because of the size and potential revenues of the pharmaceutical market, there is interest in developing instrumentation based on, and technical enhancements to, electrospray mass spectrometry.
In recent years there has been a general trend to minimize the amount of sample required for analysis and micro-electrospray ionization (micro-ESI, micro-ES) and nanospray describe two of these approaches. These two methods share a lot in common, and they are often used interchangeably. Micro-ES is a miniaturized electrospray source with the same system components as xe2x80x9cconventionalxe2x80x9d electrospray. These include a source of pumped liquid flow containing the sample for analysis, a small diameter sharp hollow needle through which liquid is pumped, and a source of high voltage to generate the spray. Nanospray relies on the electrostatic attraction of the liquid inside the needle towards an attractor counter-electrode to generate the flow rather than a pump. This characteristic makes nanospray very attractive as a means to minimize sample waste. Since electrospray, micro-ES, and nanospray are all species of a generic class referred to as electrospray they will be interchangeably referred to as electrospray in this patent.
The nature of the electrospray ionization process makes sample preparation a major consideration. The presence of solvent and buffer salts along with the sample significantly increases spectral complexity and degrades detection limits. The electrospray ionization process produces an abundance of solvent ions that give an intense mass spectral background that can severely limit identification of many compounds at trace levels in solution. Even without the solvent ions to contend with, many applications require working with complex mixtures that necessitate some degree of separation prior to mass analysis. See J. Lee, J. F. Kelly, I. Chernushevich, D. J. Harrison, and P. Thibalut xe2x80x9cSeparation and Identification of Peptides from Gel-Isolated Membrane Proteins Using a Microfabricated Device for Combined Capillary Electrophoresis/Nanoelectrospray Mass Spectrometry,xe2x80x9d Anal.Chem.2000, 72,599-609. Better methods for elimination of unwanted solvent and separation of sample ions from background are therefore needed.
Electrospray mass spectrometry (ES-MS) provides a powerful tool for structure determination of peptides, proteins. This is important, as structure to a large extent defines the function of the protein. The structural information about a protein is typically determined from its amino acid sequence. To identify the sequence, the protein is usually digested by enzymes, and the peptide fragments are sequenced by tandem mass spectrometry. Another possible way to obtain the sequence is to digest the protein and measure the molecular weights of the peptide fragments. These are the input data for a computer program which digests theoretically all the proteins being found in the data base and the theoretical fragments are compared with the measured molecular weights.
Recently, it has been noticed that Ion Mobility Spectrometry can provide useful information to an electrospray/mass-spectrometry measurement. Ion Mobility spectrometry is ordinarily an atmospheric pressure technique which is highly sensitive to the shape and size of a molecule. Protein identification thorough the combination of an IMS and mass spectrometer may eliminate the need for protein digestion, simplifying sample preparation.
Commercially available IMS systems are based on time-of-flight (TOF), i.e., they measure the time it takes ions to travel from a shutter-gate to a detector through an inert atmosphere (1 to 760 Torr.). The drift time is dependent on the mobility of the ion (i.e., its size, mass and charge) and is characteristic of the ion species detected. TOF-IMS is a technique useful for the detection of many compounds including narcotics, explosives, and chemical warfare agents. See PCT Application Ser. No. PCT/CA99/00715 incorporated herein by this reference and U.S. Pat. No. 5,420,424 also incorporated herein by this reference. In ion mobility spectrometry, gas-phase ion mobility is determined using a drift tube with a constant low field strength electric field. Ions are gated into the drift tube and are subsequently separated based on differences in their drift velocity. The ion drift velocity under these conditions is proportional to the electric field strength and the ion mobility, which is determined from experimentation, is independent of the applied field. Current spectrometers use conventionally machined drift tubes (minimum size about 40 cm3) for ion identification.
In conventional time-of-flight ion mobility spectrometers (TOF-IMS) ion identification is done in a low strength electric field (less than 1000 V/cm) where the coefficient of mobility for each ion is essentially independent of field strength [.W. McDaniel and Edward A. Mason, The mobility and diffusion of ions in gases, John Wiley and Sons, 1973].
At high electric fields, ion mobility becomes dependent upon the applied electric field strength and the ion drift velocity may no longer behave linearly with field strength. This principle is utilized in the subject of this disclosure.
The field asymmetric waveform ion mobility spectrometer (FAIMS, also known as RF-IMS) utilizes these significantly higher electric fields, and identifies the ion species based on the difference in its mobility in high and low strength electric fields.
The FAIMS spectrometer uses an ionization source, such as an ultra violet photo-ionization lamp, to convert a gas sample into a mixture of ion species with each ion type corresponding to a particular chemical in the gas sample. The ion species are then passed through an ion filter where particular electric fields are applied between electrodes to select an ion type allowed to pass through the filter. Once through the filter the ion type hits a detector electrode and produces an electrical signal. To detect a mixture of ion species in the sample, the electric fields applied between the filter electrodes can be scanned over a range and a spectrum generated. The ion filtering is achieved through the combination of two electric fields generated between the ion filter electrodes, an asymmetric, periodic, radio frequency (RF) electric field, and a dc compensation electric field. The asymmetric RF field has a significant difference between its peak positive field strength and negative field strength. The asymmetric RF field scatters the ions and causes them to deflect to the ion filter electrodes where they are neutralized, while the compensation field prevents the scattering of a particular ion allowing it to pass through to the detector. The ions are filtered in instruments on the basis of the difference in the mobility of the ion at high electric fields relative to its mobility at low electric fields. That is, the ions are separated due to the compound dependent behavior of their mobility at high electric fields relative to their mobility at low electric fields.
The FAIMS approach is based on an observation of Mason and McDaniel [.W. McDaniel and Edward A. Mason, The mobility and diffusion of ions in gases, John Wiley and Sons, 1973] who found that the mobility of an ion is affected by the applied electric field strength. Above an electric field to gas density ratio (E/N) of 40 Td (E greater than 10,700 V/cm at atmospheric pressure) the mobility coefficient K(E) has a non-linear dependence on the field. This dependence is believed to be specific for each ion species. Below are some examples from Mason and McDaniel [.W. McDaniel and Edward A. Mason, The mobility and diffusion of ions in gases, John Wiley and Sons, 1973]. The mobility for the cluster ion CO+CO increases with increasing field strength (FIG. 7-1-K-1 in reference [.W. McDaniel and Edward A. Mason, The mobility and diffusion of ions in gases, John Wiley and Sons, 1973]). For some molecular and atomic ions the coefficient of mobility can change in a more complex way. For example, for atomic ions K+, the mobility coefficient in carbon monoxide gas increases with increasing field by as much as 20%, but above E/Nxcx9c200 Td the coefficient starts to decrease (FIG. 7-1-K-3 in reference [.W. McDaniel and Edward A. Mason, The mobility and diffusion of ions in gases, John Wiley and Sons, 1973]). For some other ions for example N+, N3+ and N4+ the mobility changes very little (FIG. 7-1-H-1/2 in reference [.W. McDaniel and Edward A. Mason, The mobility and diffusion of ions in gases, John Wiley and Sons, 1973]). FIG. 1A illustrates schematically three possible ion mobility dependencies on electric field. For simplicity we will assume that the low field value of the mobility K(Emin) in a weak electric field (E approximately 102-103 V/cm) is the same for all three ion types. However, at Emax the value of the mobility coefficient K(Emax) is different for each ion type.
The field dependence of the mobility coefficient K(E) can be represented by a series expansion of even powers of E/N [18]
K(E)=K(0)[1+xcex11(E/N)2+xcex12(E/N)4+ . . . ]xe2x80x83xe2x80x83(1) 
where K(0) is the coefficient of mobility of the ion in a weak electric field, and xcex11, xcex12 are coefficients of the expansion. This equation can be simplified by using an effective xcex1(E) as shown in equation 2 [T. W. Carr, Plasma Chromatography, Plenum Press, New York and London, 1984],
K(E)≈K(0)[1+xcex1(E)]. xe2x80x83xe2x80x83(2) 
According to this expression when xcex1(E) greater than 0 the mobility coefficient K(E) increases with field strength, when xcex1(E)xcx9c0 the mobility K(E) does not change, and when xcex1(E) less than 0 then K(E) decreases with increasing field strength. An expression for the field dependent mobility coefficient can also be derived from momentum and energy balance considerations. Where the energy of the ion xcex5=3/2 kTeff can be expressed as a function of its effective temperature [18-20].                               K          ⁡                      (            E            )                          =                              v            E                    =                                    q              N                        ⁢                                          (                                  1                                      3                    ⁢                    μ                    ⁢                                          xe2x80x83                                        ⁢                    k                    ⁢                                          xe2x80x83                                        ⁢                                          T                      eff                                                                      )                                            1                /                2                                      ⁢                                          1                                  Ω                  ⁡                                      (                                          T                      eff                                        )                                                              .                                                          (        3        )            
The case where xcex1(E) less than 0 can be explained based on the model presented in equation 3, if one assumes the value of the ion neutral cross-section xcexa9(Teff) does not change significantly for rigid-sphere interactions [T. W. Carr, Plasma Chromatography, Plenum Press, New York and London, 1984, E. A. Mason and E. W. McDaniel, Transport Properties of Ions in Gases, Wiley, New York, 1988] and the reduced mass xcexc is constant. Under these conditions one finds that the mobility K(E) will decrease if the effective temperature, or energy, of the ion increases. Physically this effect has a simple explanation. When the electric field strength is increased the ions are driven harder through the neutral gas. This increases the ion neutral collision frequency, which leads to a reduced average ion velocity and a reduced ion mobility coefficient.
The rigid-sphere model however, does not explain the experimental results which show that with certain ions the mobility increases with increasing electric field (xcex1(E) greater than 0). One of the possible explanations for the increased mobility at elevated values of E/N is offered when one allows for ion de-clustering at high field strengths to occur. Ions in ambient conditions in a weak electric field generally do not exist in a free state. They are usually in cluster form (for example, MH+(H20)n) with n polar molecules such as water attached. As the electric field strength is increased the kinetic energy and consequently the effective temperature (Teff) of the ion increases due to the energy imparted between collisions. This can lead to a reduction in the level of ion clustering (reduction in n) resulting in a smaller ion cross-section xcexa9(Teff) and a smaller reduced mass xcexc for the ion. According to equation 3 then, if do to de-clustering the cross-section and reduced mass decrease in a sufficient manner to offset the increase in Teff the case where xcex1(E) greater than 0 can be explained.
The third case when xcex1(E)xcx9c0 can be explained by a decrease in ion cross section due to de-clustering which is offset by an increase in the effective temperature of the ion. This results in no net change to the mobility coefficient of the ion.
The mechanism of operation of the FAIMS for ion filtering is described in the following. Consider three kinds of ions with different mobility coefficient dependencies on electric field (i.e., xcex1(E) greater than 0, xcex1(E) less than 0, xcex1(E)xcx9c0) which are formed, due to local ionization of neutral molecules, at the same location in a narrow gap between two electrodes, as shown on FIG. 1B. A stream of carrier gas transports these ions longitudinally down the drift tube between the gap. If an asymmetric RF electric field is then applied to the electrodes the ions will oscillate in a perpendicular direction to the carrier gas flow, in response to the RF electric field, while moving down the drift tube with the carrier gas. A simplified asymmetric RF electric field waveform (FIG. 1C) with maximum field strength |Emax| greater than 10,000 V/cm and minimum field strength |Emin| less than  less than |Emax| is used here to illustrate the operation principle of the RF-IMS. The asymmetric RF waveform is designed such that the time average electric field is zero and
|Emax|t1=|Emin|t2=xcex2.xe2x80x83xe2x80x83(1) 
t1 is the portion of the period where the high field is applied and t2 is the time the low field is applied. xcex2 is a constant corresponding to the area under-the-curve in the high field and low field portions of the period. The ion velocities in the y-direction are given by
Vy=K(E)E(t).xe2x80x83xe2x80x83(2) 
Here K is the coefficient of ion mobility for the ion species and E is the electric field intensity, in this case entirely in the y-direction. If the amplitude of the positive polarity RF voltage pulse (during t1) produces an electric field of strength greater than 10,000 V/cm then the velocity towards the top electrode
Vup=Kup|Emax|xe2x80x83xe2x80x83(3) 
will differ for each of the ion species (FIG. 1B) since, as shown in FIG. 1A, the coefficient of mobility Kup for each ion at the high field condition is different. The ions with xcex1(E) greater than 0 will move faster and ions with xcex1(E) less than 0 will have the smallest velocity, therefore, the slope of each ion""s trajectory will also differ. In the next portion of the period (t2), once the polarity of the RF field has switched, all three ion types will begin moving with the same velocity
Vdown=K(Emin)|Emin|xe2x80x83xe2x80x83(4) 
down towards the bottom plate. In this low field strength condition (see FIG. 1A) all three ion types will have the same mobility coefficient Kdown. Therefore, all three ion trajectories will have the same slope in this portion of the period (FIG. 1B).
The ion displacement from its initial position in the y-direction is the ion velocity in the y-direction Vy multiplied by the length of time xcex94t the field is applied
xcex94y=Vyxcex94t.xe2x80x83xe2x80x83(5) 
In one period of the applied RF field the ion moves in both the positive and negative y-directions. By substituting equation 2 into equation 5 the average displacement of the ion over one period of the RF field can be written as
xcex94yRF=Kup|Emax|t1xe2x88x92Kdown|Emin|t2.xe2x80x83xe2x80x83(6) 
Using equation 1 this expression can be re-written as
xcex94yRF=xcex2(Kupxe2x88x92Kdown)=xcex2xcex94K.xe2x80x83xe2x80x83(7) 
Since xcex2 is a constant determined by the applied RF field, the y-displacement of the ion per period of the RF field T=t1+t2 depends on the change in mobility of the ion between its high and low field conditions. Assuming the carrier gas only transports the ion in the z-direction. The total ion displacement Y (in the y-direction) from its initial position (due to the electric field) during the ions residence time tres between the ion filter plates can be expressed as                     Y        =                                                            Δ                ⁢                                  xe2x80x83                                ⁢                                  y                  RF                                                            (                                                      t                    1                                    +                                      t                    2                                                  )                                      ⁢                          t              res                                =                                                    β                ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                K                            T                        ⁢                          t              res                                                          (        8        )            
The average ion residence time inside the ion filter region is given in equation 9. A is the cross-section area of the filter region, L is the length of the ion filter electrodes, V is the volume of the ion filter region V=AL, and Q is the volume flow rate of the carrier gas.                               t          res                =                              AL            Q                    =                                    V              Q                        .                                              (        9        )            
Substituting equation 9 into equation 8, noting from equation 1 that xcex2=|Emax|t1 and defining the duty cycle of the RF pulses as D=t1/T. The equation for displacement of the ion species, equation 8, can be re-written as                     Y        =                              Δ            ⁢                          xe2x80x83                        ⁢                          KE              max                        ⁢            VD                    Q                                    (        10        )            
where Y is now the total displacement of the ion in the y-direction based on the average ion residence time in the ion filter region. From equation 10 it is evident that the vertical displacement of the ions in the gap are proportional to the difference in coefficient of mobility between the low and high field strength conditions. Different species of ions with different xcex94K values will displace to different values of Y for a given tres. All the other parameters including the value of the maximum electric field, the volume of the ion filter region, the duty cycle and the flow rate, to first order are essentially the same for all ion species.
When a low strength DC field (|Ec| less than |Emin| less than  less than |Emax|) is applied in addition to the RF field, in a direction opposite to the average RF-induced (y-directed) motion of the ion, the trajectory of a particular ion species can be xe2x80x9cstraightenedxe2x80x9d, see FIGS. 1D(1), 1D(2), 1D(3). This allows the ions of a particular species to pass unhindered between the ion filter electrodes while ions of all other species are deflected into the filter electrodes. The DC voltage that xe2x80x9ctunesxe2x80x9d the filter and produces a field which compensates for the RF-induced motion is characteristic of the ion species and is called the compensation voltage. A complete spectrum for the ions in the gas sample can be obtained by ramping or sweeping the DC compensation voltage applied to the filter. The ion current versus the value of the sweeping voltage forms the RF-IMS spectra. If instead of sweeping the voltage applied to one of the ion filter electrodes, a fixed DC voltage (compensation voltage) is applied, the spectrometer will work as continuous ion filter allowing only one type of ion through.
In PCT application Ser. No. PCT/CA99/00715, an electrospray ionization chamber or electrospray source is used to create ions which are ultimately transported to an analytical region which is subject to both a high frequency voltage asymmetric waveform and a DC offset voltage.
It is therefore an object of the present invention to provide method and apparatus for improved detection of compounds using field asymmetric waveform ion mobility.
Objects of the invention are achieved in practice of field asymmetric ion mobility spectrometers and novel improvements, particularly in three areas: 1) sample preparation and introduction, 2) ion filtering, and 3) output and signal collection.
Embodiments of the invention feature combinations of various aspects, including use of a FAIMS ion filter to filter ions where control of which ions are filtered is achieved by control of a variable DC compensation signal in addition to a high field asymmetric waveform radio frequency signal or use of a FAIMS filter where the control of which ions are filtered is achieved by varying the wavelength, frequency, amplitude, period, duty cycle or the like of the high field asymmetric waveform radio frequency signal; use of a planar FAIMS filter which uses insulating substrates to very accurately control the gap between the ion filter electrodes and ensure the ion filter electrodes are parallel, this allows very reproducible fields to be obtained which results in a higher resolution spectrometer; use of a planar FAIMS filter where the insulating spacers overlap the edges of the filter electrodes, which results in a higher resolution FAIMS with more accurate identification of compounds since all the sample is forced to pass between the ion filters and no ions can bypass the filter electrodes and still reach the detector electrode.
In use with a spray source, such as electrospray, where desolvation of the ions is very important in order to obtain reliable, reproducible spectra, desolvation is achieved. Desolvation electrodes may be included to assist in desolvation, where enhanced desolvation is achieved by applying symmetric RF signals to the desolvation electrodes. The RF signals provide energy to the ions which raises their effective temperature and helps to enhance the desolvation process.
Desolvation electrodes can also be used to control the level of ion clustering in gas samples from electrospray and from other than electrospray sources. Control of ion clustering can permit more repeatable measurements and also can provide additional information on the ions being detected.
A novel embodiment of the invention relates to the sample preparation section. This embodiment incorporates the use of an electrospray head and the use of an attraction electrode which is separated from the ion filter electrodes. The advantage of separating the attraction electrode from the ion filter electrodes is that this allows freedom in applying a different potential to the attraction electrode relative to the ion filter electrodes, and this allows optimization of the electrospray conditions and ion introduction conditions into the FAIMS. This separation of attraction electrode from the ion filter electrodes can also be realized in cylindrical FAIMS configurations.
Additionally, guiding electrodes can be provided and allow further optimization of ion injection into the ion filter. In a further embodiment of the invention the electrospray assembly can be attached to one of the substrates of the FAIMS and guiding electrodes are used to guide the ions into the ionization region. The guiding electrodes can be a freestanding structure attached or connected to or near one of the substrates of the FAIMS. The assembly can have a counter gas flow to enhance desolvation.
The invention also features the realization of the concept that a time-of-flight measurement can be combined with a FAIMS approach using electrospray to provide improved identification of the ion species through the additional information provided by the time-of-flight measurement. The time it takes the ion to travel from the orifice of the FAIMS to the detector can be measured. This can be achieved through the independent control of the attraction and guiding electrode potentials. For example, initially the attraction electrode potential is adjusted so that no ions make it into the drift region, but rather are collected at the guiding electrodes. Then the attraction electrode is pulsed so that some ions can make it into the ionization region and into the ion filter. Now the time it takes the ions to travel from the ionization region to the detector can be measured, and this provides additional discriminating information on the identity of the ion.
A novel aspect of the invention is the concept of formation of electrodes on an insulating or insulated substrate where the insulating substrate can form a housing. This approach provides significant advantages in simplification of device construction. It allows low cost, mass producible processes to be used such as micromachining and multichip modules which can result in low cost, miniature sensors.
In the output section, the embodiments of the FAIMS proposed are the first to have output sections with the ability to detect multiple ion species simultaneously such as a positively and negatively charged ion.
Since sample analysis in the FAIMS is generally performed in the gas phase, liquid samples require conversion from the liquid to the gas phase. In a preferred embodiment, the electrospray method (which we define as encompassing xe2x80x9cconventionalxe2x80x9d, micro and/or nanospray) is used to convert a liquid sample into gas phase ions. Preferably the ions streaming out of the electrospray tip are submitted to a planar FAIMS device. In a preferred practice of the invention, all the functions of sample preparation, ionization, filtering and detection are performed on a single xe2x80x9cchipxe2x80x9d.
In another embodiment, the electrospray-FAIMS is applied as a filter to a mass spectrometer. The FAIMS coupled to the mass spectrometer provides enhanced resolution, better detection limits, ability to extract shape and structure information of the molecules being analyzed, molecules can include bio-molecules such as proteins and peptides. The FAIMS technique is based on ion mobility, where ion filtering and identification is highly dependent on the size and shape of the ion. This information is of great interest in genomics and proteomics research (i.e., pharmaceutical industry) since the shape of a protein to a large extent determines its functionality and therefore FAIMS filtering can be applied as a low cost high volume method of protein characterization. A particular embodiment includes a disposable FAIMS filter chip which is plugged into a carrier mounted on the inlet of a mass spectrometer. The FAIMS-electrospray device can also provide structural (conformation) information about the molecule being analyzed and sequence information not obtainable simply with electrospray-mass spectrometry. In addition the FAIMS allows discrimination between isomers (molecules with the identical mass but which differ in their shape) which cannot be identified using electrospray-mass spectrometry alone.
In a particular embodiment, the electrospray-FAIMS forms a filter and detection system in a single housing. The electrospray-FAIMS configuration of the present invention can be used as a standalone detector for liquid sample analysis or as the front end to a mass spectrometer. The present invention also has application to other liquid separation techniques such as liquid chromatography, high pressure liquid chromatography, and capillary electrophoresis. A preferred embodiment of the invention includes a planar FAIMS apparatus where in one embodiment the device is integrated with an electrospray ionizing source on a common housing or substrate and is coupled to a mass spectrometer. Alternative practices of the invention may include cylindrical or coaxial FAIMS devices.
Embodiments of the invention enable filtering of molecules after they have been ejected from a source, such as from an electrospray tip or a capillary electrophoresis outlet, and have been ionized prior to filtering via a FAIMS filter, and detected via an internal detector or via a mass spectrometer or other detector. In one practice of the invention, micromachining (MEMS) processing enables integration of an electrospray tip with a FAIMS filter into a simple device and results in a precise yet compact analytical system for accurate, highly repeatable, liquid sample evaluation. In another practice of the invention, portable, miniature, low cost, bio-sensors for biological agent detection which use an integrated electrospray-FAIMS chip are possible; preferably they are prepared using micromachining fabrication techniques. In one embodiment an atmospheric pressure chemical ionization (APCI) device is achieved with a FAIMS filter used as a prefilter to a mass spectrometer.
Prior to the present invention, conventional machining led to high cost of fabrication and poor reproducibility from FAIMS device to FAIMS device. Furthermore, prior art cylindrical FAIMS geometry either limits collection efficiency when interfacing to a mass spectrometer, or permits both sample neutrals and sample ions to enter the mass spectrometer, resulting in more complex spectra. Advantageously in practice of the invention, ion filtering is performed after sample ionization, therefore buffer salt and solvent ions, which are invariably generated in the electrospray process, are separated from the bio-molecules of interest. This provides significantly simpler mass spectra and improves the detection limits and identification of the bio-molecules.
Combination of electrospray with a new FAIMS filter device enables analytical detection devices with greatly enhanced sensitivity and resolution. In some cases the ability is provided to resolve compounds that could not be identified without the FAIMS present. Combination of electrospray with a prior art FAIMS filter devices raises issues of sample to sample contamination when running low concentration samples through the device for high throughput low cost sample analysis, but these are overcome in practice of the present invention.
The new FAIMS of the invention is a low cost, a volume manufacturable, small and compact, spectrometer based on differential ion mobility. The present invention, particularly configured using high volume manufacturing techniques, such as MEMS fabrication techniques which includes ceramic packaging, PC board manufacturing techniques or plastic processing, offers several additional advantages over prior devices. The volume manufacture techniques result in low cost devices that can be made disposable, thus avoiding the problem of sample cross contamination. These chips will be available to any laboratory using a mass spectrometer for biological molecule identification as a FAIMS interface filter. Such a filter includes the FAIMS interface chip which can plug into an interface fixture which contains, filtering electronics. The electrospray tip or electrophoresis chips can be integrated with (fabricated as part of) the FAIMS chip. The MEMS approach is not required but is preferred and renders high reliability and repeatability in volume manufactured FAIMS chips; this lowers their cost and enables disposable devices. This disposability avoids contamination from one sample to the next, which is invaluable for tests performed subject to, for and/or by regulatory agencies like the EPA and FDA where contamination is a concern.
In one embodiment of the present invention, a planar MEMS FAIMS chip was fabricated in which ions are focused into a mass spectrometer and collection efficiency is close to 100%. In this embodiment, no ion injection is required into the FAIMS ion filter region. The device is micromachined on a planar surface. This enables easy integration with onboard heaters to minimize ion clustering. It can be easily integrated with micromachined or conventional electrospray tips and/or micromachined electrophoresis chips. This is a simplified design with reduced fabrication requirements, and can be configured to use only a single gas flow channel.
Micromachining provides for excellent reproducibility in the manufacture and performance of the filters. This is critical so that test results are consistent from one device to the next and from one laboratory to the next. Micromachining enables new configurations of FAIMS filter chips which cannot be made any other way. These new configurations are simpler and more efficient at delivering ions to the mass spectrometer and filtering unwanted ions.
A MEMS FAIMS drift tube has been successfully fabricated and characterized. High spectrometer sensitivity and ability to resolve chemicals not separated in conventional TOF-IMS has been demonstrated. The MEMS FAIMS enables the realization of miniature, low cost, high sensitivity, high reliability chemical detectors. The FAIMS spectrometer of the invention has also been demonstrated as a pre-filter to a mass spectrometer. The new FAIMS/MS combination allows better resolution of complex mixtures.
Portable, miniature, low cost, bio-sensors for biological agent detection which use an integrated electrospray-FAIMS chip are possible using microfabrication methods such as micromachining s because of the size reduction and cost reductions enabled by this technology and enabled manufacture. These instruments will have many uses, including availing high quality bio-analysis in the field. For example, a person suspected of being exposed to a bio-agent will supply a drop of blood to the instrument. The blood will be mixed with a buffer solution, processed, and introduced via the electrospray nozzle into the FAIMS where the ions will be analyzed. If a particular bio-molecule is detected an alarm will be set off. As well, micromachining enables new configurations of FAIMS filter chips which are not otherwise available. For example, the planar FAIMS disclosed in this patent. These new configurations are simpler and more efficient at delivering ions to the mass spectrometer and filtering unwanted ions. In a preferred embodiment, the Electrospray and FAIMS form an atmospheric pressure chemical ionization (APCI) prefilter and analyte filter and detection system in a single housing. The new FAIMS-APCI provides high performance and low cost, volume manufacturable, small and compact, ion mobility spectrometer.
In an embodiment of the invention, a breakthrough can be attributed to providing a multi-use housing/substrate/packaging that simplifies formation of the component parts and resulting assembly. Additional features include the possibility to use the substrate as a physical platform to build the filter upon and to give structure to the whole device, to use the substrate as an insulated platform or enclosure that defines the flow path through the device, and/or use the substrate to provide an isolating structure that improves performance. A spacer can be incorporated into the device, which provides both a defining structure and also the possibility of a pair of silicon electrodes for further biasing control. Multiple electrode formations and a functional spacer arrangement can be utilized which improve performance and capability. Filtering employs the FAIMS asymmetric periodic voltage applied to the filters along with a control component, and this component can be a bias signal or voltage or may be supplied simply otherwise, such as by control of the duty cycle of the same asymmetric signal and which removes the need for the DC compensation circuit. This compact arrangement enables inclusion of a heater for purging ions, and may even include use of the existing electrodes, such as filter or detector electrodes, for heating/temperature control.
Embodiments of the invention include a field asymmetric ion mobility spectrometer apparatus and some preferred embodiments include a sample preparation and introduction section, ion filtering section, and an output section and a control section, the filter comprising a FAIMS ion filter for filtering ions. Embodiments of the invention may variously include: planar FAIMS filter which uses insulating substrates to very accurately control the gap between the ion filter electrodes and ensure the ion filter electrodes are parallel, this allows very reproducible fields to be obtained which results in a higher resolution spectrometer; a FAIMS filter where the insulating spacers overlap the edges of the filter electrodes. This results in a higher resolution FAIMS with more accurate identification of compounds since all the sample is forced to pass between the ion filters and no ions can bypass the filter electrodes and still reach the detector electrode; an electrospray head and providing desolvation of the ions via desolvation electrodes; enhanced desolvation is achieved by applying symmetric RF signals to the desolvation electrodes; the RF signals provide energy to the ions which raises their effective temperature and helps to enhance the desolvation process; wherein desolvation electrodes for control of the level of ion clustering in gas samples, for more repeatable measurements and providing additional information on the ions being detected, are provided in a practice of the invention.
A novel aspect may include the concept of formation of electrodes on an insulating or insulated substrate where the insulating substrate can form a housing, and providing significant advantages in simplification of device construction, with low cost, mass producible processes to be used such as micromachining and manufacture of multichip modules which can result in low cost, miniature sensors using FAIMS architecture; within the output section the ability to detect multiple ion species simultaneously such as a positively and negatively charged ion; further incorporating the use of an electrospray head and the use of an attraction electrode which is separated from the ion filter electrodes to permit applying a different potential to the attraction electrode relative to the ion filter electrode(s), and this allows optimization of the electrospray conditions and ion introduction conditions into the FAIMS device. The FAIMS device may be cylindrical-type, planar-type, or otherwise. Guiding electrodes can allow further optimization of ion injection into the ion filter.
It is also possible to form a time-of-flight measurement device combined with the FAIMS device using electrospray to provide improved identification of the ion species through the additional information provided by the time-of-flight measurement; the time it takes the ion to travel from the orifice of the FAIMS to the detector can be measured, which can be achieved through the independent control of the attraction and guiding electrode potentials; wherein the electrospray assembly can be attached to one of the substrates of the FAIMS and guiding electrodes are used to guide the ions into the ionization region; a counter gas flow enhances desolvation; wherein the guiding electrodes can be a freestanding structure attached or connected to or near one of the substrates of the FAIMS; wherein control of which ions are filtered is achieved by control of a variable DC compensation signal in addition to a high field asymmetric waveform radio frequency signal, or control of which ions are filtered is achieved by varying an aspect of the field such as the duty cycle, amplitude or frequency of the high field asymmetric waveform radio frequency signal, among others.
All the functions of sample preparation, ionization, filtering and detection can be performed on a single chip or workpiece of the invention; wherein an electrospray-FAIMS is applied as a filter to a mass spectrometer; a chip carrier and a disposable FAIMS filter chip which is plugged into the carrier, the carrier enable for mounting on the inlet of a mass spectrometer; wherein an electrospray-FAIMS forms a filter and detection system in a single housing; wherein electrospray-FAIMS configuration of the present invention can be used as a standalone detector for liquid sample analysis or as the front end to a mass spectrometer; wherein present invention also has application to other liquid separation techniques such as liquid chromatography, high pressure liquid chromatography, and capillary electrophoresis; wherein a preferred embodiment of the invention includes a FAIMS apparatus where in one embodiment the FAIMS device is integrated with an electrospray ionizing source on a common housing or substrate and is coupled to a mass spectrometer; wherein embodiments of the invention enable filtering of molecules after they have been ejected from a source, such as from an electrospray tip or a capillary electrophoresis outlet, and have been ionized prior to filtering via a FAIMS filter, and detected via an internal detector, or via a mass spectrometer or other detector, and in a practice of the invention, micromachining (MEMS) processing enables integration of an electrospray tip with a planar field asymmetric waveform ion mobility spectrometer filter into a simple unit/device and results in a precise yet compact analytical system for accurate, highly repeatable, liquid sample evaluation, or in another practice of the invention, portable, miniature, low cost, bio-sensors for biological agent detection which use an integrated electrospray-FAIMS chip are possible, possibly prepared using micromachining fabrication techniques; wherein the FAIMS part is planar and forms an atmospheric pressure chemical ionization (APCI) prefilter to a mass spectrometer; wherein ion filtering is performed after sample ionization, therefore buffer salt and solvent ions, which are invariably generated in the electrospray process, are separated from the bio-molecules of interest and this provides significantly simpler mass spectra and improves the detection limits and identification of the bio-molecules; wherein combination of electrospray with a FAIMS filter device enables analytical detection devices with greatly enhanced sensitivity and resolution and wherein in some cases the ability is provided to resolve compounds that could not be identified without the FAIMS present, and wherein combination of electrospray with a prior art FAIMS filter devices raises issues of sample to sample contamination when running low concentration samples through the device, high throughput low cost sample analysis, but these are overcome in practice of the present invention.
A mass spectrometer is directly coupled to the exhaust port at the end of the drift tube, wherein a baffle may be placed to regulate the velocity of waste gas flow stream relative to the velocity of drift gas flow stream, in a practice of the invention. Various sample preparation sections can be used, whether simply a port to draw in ambient air samples, or electrospray, gas chromatograph, liquid chromatograph, or the like. A split gas flow may be used to prevent clustering and allows better identification of ion species.
The relationship between the amount of monomer and cluster ions for a given ion species is dependent on the concentration of sample and the particular experimental conditions (e.g., moisture, temperature, flow rate, intensity of RF-electric field). In a practice of the invention, both monomer and cluster states are detected to provide useful information for chemical identification. In one example, a planar two channel FAIMS is used to achieve this result, wherein a curtain gas is applied to sample neutrals and they are prevented from entering the second channel xe2x80x9cIIxe2x80x9d and ions in the monomer state can be investigated. In another embodiment, curtain gasses may flow in the same direction and exhaust at an orifice or in opposite directions while guiding electrodes are included to guide the ions into the second channel xe2x80x9cIIxe2x80x9d and an attraction electrode is also used to attract ions into channel xe2x80x9cIIxe2x80x9d, such that when the curtain gas is turned off ions in the cluster state may be observed since sample neutrals and sample ions may now be drawn into channel xe2x80x9cIIxe2x80x9d using a pump. The output section may be connected to a mass spectrometer.
A method of the invention includes coupling a FAIMS device to a mass spectrometer, for providing enhanced resolution, better detection limits, ability to extract shape and structure information of the molecules being analyzed, molecules can include bio-molecules such as proteins and peptides, the FAIMS technique being based on ion mobility, where ion filtering and identification is highly dependent on the size and shape of the ion, the FAIMS-electrospray device providing structural (conformation) information about the molecule being analyzed and sequence information not obtainable simply with electrospray-mass spectrometry and also allowing discrimination between isomers (molecules with the identical mass but which differ in their shape) which cannot be identified using electrospray-mass spectrometry alone.
Embodiments of the invention feature a multi-functional use of the FAIMS substrates. The substrates are platforms (or a physical support structures) for the precise definition and location of the component parts or sections of the device. The substrates form a housing, enclosing the flow path with the filter and perhaps the detector, as well as other components. This multi-functional design reduces parts count while also precisely locating the component parts so that quality and consistency in volume manufacture can be achieved. The smaller device also has unexpected performance improvements, perhaps because of the smaller drift tube and perhaps also because substrates also perform an electronic isolation function. By being insulating or an insulator (e.g., glass, ceramic, plastic), the substrates can be a direct platform for formation of components, such as electrodes, with improved performance characteristics.
Embodiment of the invention may be cylindrical or planar, or the like. In disclosed embodiments, use of substrates as a support/housing does not preclude yet other xe2x80x9chousingxe2x80x9d parts or other structures to be built around the device. For example, it might be desirable to put a humidity barrier over the device. As well, additional components, like batteries, can be mounted to the outside of the substrate/housing, e.g., in a battery enclosure. Nevertheless, embodiments of the presently claimed invention can provide a substrate insulation function, support function, multi-functional housing function, as well as other functions.
The insulative or insulated substrate/flow path invention achieves excellent performance in a simplified structure. The use of an electrically insulated flow path in a FAIMS device enables the applied asymmetric periodic voltage to be isolated from the output part (e.g., from the electrodes of the detector), where detection takes place. This reduction is accomplished because the insulated substrates provide insulated territory between the filter and detector in the flow path, and this spacing in turn advantageously separates the filter""s field from the detector. The less noisy detection environment means a more sensitive FAIMS device.
Moreover, by forming the electrodes on an insulative substrate, the ion filter electrodes and detector electrodes can be positioned closer together which unexpectedly enhances ion collection efficiency and favorably reduces the device""s mass that needs to be regulated, heated and controlled. This also reduces power requirements. Furthermore, use of small electrodes reduces capacitance which in turn reduces power consumption. As well, tightly spaced electrodes lends itself to a mass production process, since the insulating surfaces of the substrates are a perfect platform for the forming of such electrodes.
Embodiments of the claimed invention result in FAIMS devices that achieve high resolution, fast operation and high sensitivity, yet with a low parts count and in configurations that can be cost-effectively manufactured and assembled in high volume. Quite remarkably, packaging is very compact for such a capable FAIMS device, with sensitivity in the range of parts per billion or trillion. In addition, the reduced real estate of this smaller device leads to reduced power requirements, whether in sensing ions or in heating the device surfaces, and can enable use of a smaller battery. The benefits of the simplified and compact FAIMS spectrometer according to the invention requires typically as little as one second (and even less) to produce a complete spectrum for a given sample. No FAIMS system has ever been taught or disclosed in the prior art that can achieve such beneficial results.